There has been considerable effort devoted to improving the sensitivity of silver halide crystals to actinic radiation and thereby increasing the sensitivity of the photographic elements in which they are contained. In this regard, photographic chemists have attempted to vary the components of, or the processes for making, silver halide emulsions. One particularly preferred means to improve sensitivity has been to chemically sensitize photographic emulsions with one or more compounds containing labile atoms of gold, sulfur, selenium or the like. Examples of chemically sensitized photographic silver halide emulsion layers are described in, for example, Research Disclosure, Item No. 308119, December 1989, Section III, and the references listed therein. (Research Disclosure is published by Kenneth Mason Publications Ltd, Dudley Annex, 12a North Street, Emsworth, Hampshire PO 10 7DQ, England.)
Many gold sensitizers have been described. For example, U.S. Pat. No. 3,503,749 describes the use of water soluble Au(I) thiolate salts comprising one Au atom ligated to one sulfur containing ligand; U.S. Pat. No. 5,220,030 teaches the use of Au(I) compounds with bis mesoionic heterocycles; U.S. Pat. No. 5,252,455 and U.S. Pat. No. 5,391,727 disclose the use of Au(I) macrocyclic cationic sensitizers; U.S. Pat. No. 5,049,484 teaches the use of Au(I) sensitizers having a Au atom ligated to the nitrogen atom of heterocyclic rings. U.S. Pat. No. 5,620,841 discloses the use of gelatin dispersions of a Au(I) thiosulfonato sensitizer with two different ligands at least one of which is mesoionic; and U.S. Pat. No. 5,700,631 teaches the use of gelatin dispersions of Au(I) thiosulfonato sensitizers with two different ligands at least one of which is a thioether group. JP 8069075 discusses the use of organic gold sulfide compounds in the sensitization to give low fogging and high contrast silver halide photographic materials. However, all of the above compounds have one or more disadvantages such as lack of water solubility, difficulty of synthesis or poor stability.
One common chemical sensitizer used in the sensitization of silver halide emulsions is aurous sulfide, which is made as a colloidal gelatin dispersion, the exact composition of which is not well characterized. This gold sulfide dispersion can give rise to lot-to-lot variability and undesirable and inconsistent sensitometric performance. The source of this variability may come from side reactions in the preparation of this highly insoluble solid since these reactions produce species which may be photographically active. Further, because of the highly insoluble nature of gold sulfide, most of the sensitizer added is in fact unused during the sensitization. The remaining sensitizer left in the gel/silver halide matrix can affect sensitometry.
The bis Au(I) mesoionic heterocycles, e.g. bis(1,4,5-trimethyl-1,2,4-triazolium-3-thiolate) gold (I) tetrafluoroborate, while being very useful sensitizers, are somewhat lacking in solution stability. Further, for the mesoionic triazolium sensitizers, multiple steps and recrystallizations are required in the preparation of the starting material bis(tetramethylthiourea) Au(I) tetrafluoroborate. Synthesis of the gold ligand 1,4,5-trimethyl-1,2,4-triazolium-3-thiolate is difficult, and the preparation of the mesoionic triazolium sensitizer is limited to small batches. Finally, the limited solubility of the mesoionic triazolium sensitizers requires the use of a large volume of water for dissolution.
Aurous dithiosulfate, Au(I)(SSO.sub.3).sub.2, a gold sensitizer that is water soluble, has its limitations for sensitization. One of the limitation is that aurous dithiosulfate contains a labile sulfur atom which also sulfur sensitizes the silver halide photographic emulsion. Further, because of the sulfur and gold composition of the chemical, the gold to sulfur sensitization ratio is always limited to 1:2. Thus it would not be possible to use this sensitizer alone for a gold only sensitization or for any other sensitization where the ratio of gold to sulfur desired is different from 1:2.
U.S. Pat. Nos. 5,912,112, 5,945,270 and 6,034,249 describe Au(I) complexes comprising organomercapto ligands. U.S. Pat. Nos. 5,945,270 and 6,034,249 in particular describe water soluble symmetrical bis organomercapto Au(I) complexes which provide numerous advantages. They are highly effective sensitizers for silver halide emulsions. They are also highly water soluble. Because of the water solubility of these complexes, the use of costly and time consuming preparation of gel dispersions is unnecessary. Further, there is no need to use large volumes of water for dissolving the complexes. Additionally, they are easily manufactured from readily available starting materials. Use of such organomercapto gold complexes with common sulfur sources such as thiosulfate in the chemical sensitization of silver halide emulsions employed in color reversal elements, however, has also been found to result in an undesirable speed variability dependent upon sensitization temperatures.
In the art of chemical sensitization, it is known that the emulsion sensitivity and fog propensity are strongly dependent on the sensitization temperature. The use of higher temperature often leads to high fog. Lower temperature may result in lower sensitivity. Such variations in emulsion performance as a result of temperature variations not only leads to poor quality of emulsion performance, but can lead to waste and increase the cost of manufacturing high quality products. Thus, there is a need for sensitizing silver halide emulsions that have minimal variations due to temperature fluctuations during sensitization. Hence, it would be desirable to provide color reversal elements comprising emulsions chemically sensitized with organomercapto Au(I) complexes and a sulfur source, whereby a reduced sensitivity to temperature, known as finish robustness, is achieved.